Disk algebra

In mathematics, specifically in functional and complex analysis, the disk algebra A(D) (also spelled disc algebra) is the set of holomorphic functions

ƒ : D → ${\displaystyle \mathbb {C} }$

(where D is the open unit disk in the complex plane ${\displaystyle \mathbb {C} }$) that extend to a continuous function on the closure of D. That is,

${\displaystyle A(\mathbf {D} )=H^{\infty }(\mathbf {D} )\cap C({\overline {\mathbf {D} }}),}$

where H^∞(D) denotes the Banach space of bounded analytic functions on the unit disc D (i.e. a Hardy space).

When endowed with the pointwise addition (f + g)(z) = f(z) + g(z) and pointwise multiplication (fg)(z) = f(z)g(z), this set becomes an algebra over C, since if f and g belong to the disk algebra, then so do f + g and fg.

Given the uniform norm

${\displaystyle \|f\|=\sup {\big \{}|f(z)|\mid z\in \mathbf {D} {\big \}}=\max {\big \{}|f(z)|\mid z\in {\overline {\mathbf {D} }}{\big \}},}$

by construction, it becomes a uniform algebra and a commutative Banach algebra.

By construction, the disc algebra is a closed subalgebra of the Hardy space H^∞. In contrast to the stronger requirement that a continuous extension to the circle exists, it is a lemma of Fatou that a general element of H^∞ can be radially extended to the circle almost everywhere.